IP Networks Over Heterogeneous Embedded Serial Links

Abstract

The Internet Protocol (IP) provides a number of key benefits to networked devices: it serves as a "narrow waist" enabling functional modularity by decoupling lower-layer devices from application behavior, it provides a notion of transitive connectivity and a number of standardized methods to achieve it, and most importantly, it is ubiquitous, enabling almost all networked applications to mutually communicate.

Many embedded microcontrollers cannot take advantage of the benefits of IP because they lack the dedicated networking hardware which is as a practical matter required to interact with nontrivial networks. I observe that multihop point-to-point IP networks can in principle be constructed over the communication media that microcontrollers commonly do have, such as UARTs, I2C, SPI, and CAN bus, but software support is lacking to make this networking approach accessible.

Therefore, this thesis develops and evaluates interstice, a platform-independent, open-source software library designed to enable the flexible implementation of modular packet forwarders in userspace. It can be used to interconnect devices and their IP stacks across a variety of conventional and unconventional links. Interstice exposes a reprogrammable, dynamically-updatable packet-forwarding strategy, enabling forwarder nodes in principle to act as hubs, bridges, full routers, or implement firewalls or NAT, as application requirements and platform constraints permit.

This approach enables benefits for modular, networked systems of microcontrollers which need to talk to the outside world: using IP enables internal microcontrollers to communicate with external devices such as PCs and smartphones without the need for application gateways. Further, to the extent that such networks are runtime-reconfigurable, features of IP such as address assignment, dynamic routing, and link-agnosticity can be incredibly beneficial.

Interstice is evaluated here primarily against networks of various types of serial links (UART, I2c, CAN) speaking PPP, selected to demonstrate utility of the approach to connect embedded devices lacking dedicated networking peripherals, and further that link drivers can be specialized to take advantage of the specific characteristics of each link. The approach is showcased in application scenarios including a networked milling machine, and is analyzed for a number of performance metrics.